1. Explicit Congestion Notification (ECN) Qi (Gill) Wang gillwang@udel.edu CISC 856 – TCP/IP, Fall 2012 Special thanks to: Dr. Paul Amer Guna Ranjan, Justin Yackoski, Namratha Hundigopal and Preethi Natarajan for slides

2. Overview Congestion control TCP congestion control Active Queue Management (AQM) Explicit Congestion Notification (ECN) ECN in IP and TCP Example Advantages and implementations

3. Routers have buffers that hold the packets before and after processing Buffers are finite, so it is possible for more packets to arrive at a router than the router can buffer The router becomes overloaded, and congestion occurs Typical effects of congestion Queue delay Packet loss Blocking of new connections Congestion Sender Receiver Packets are dropped

4. TCP Congestion Control TCP Treats the network as a “black-box” Relies on packet loss as the indication of congestion Congestion detection Three duplicate ACKs RTO timer times out Congestion avoidance Multiplicative decrease Reaction after congestion has already happened! Congestion has already happened!

5. Drawbacks of TCP Congestion Control Not helpful for delay-sensitive flows The increased latency of the packet is caused by the need to retransmit the packet after a loss Sender 4 Sender 3 Sender 2 Sender 1 Undesirable global synchronization All flows reduce the sending rate simultaneously  channel is under-utilized All flows start retransmission/increasing the sending rate in a similar fashion  congestion occurs again Lockout The shared resource is unfairly consumed exclusively by a small number of flows The remaining flows are denied access to the resource

6. Active Queue Management (AQM) Routers detect congestion before the buffer is full Incipient congestion indicated by the average queue size The router provides an indication to the hosts before congestion loss really happens Mark packets  ECN Sender Receiver th min th max

7. Explicit Congestion Notification (ECN) ECN is an AQM mechanism Sender responds to ECN indication the same as to a packet drop Halve cwnd Support from two layers Network layer: signaling between routers and hosts IP Transport layer: host-to-host signaling, congestion control TCP, SCTP

8. Differentiated Services Codepoints 6 bits Unused 2 bits ECN Bits in IP Header VER 4 bits HLEN 4 bits DS 8 bits Total Length 16 bits Identification 16 bits Flags 3 bits Fragmentation offset 13 bits Time to Live 8 bits Protocol 8 bits Header Checksum 16 bits Source IP Address 32 bits Destination IP Address 32 bits Options (if any) Differentiated Services Codepoints 6 bits ECT 1 bit CE 1 bit ECT: ECN Capable Transport CE: Congestion Experienced ECTCE 00Not-ECT 01ECT (1) 10ECT (0) 11CE

9. ECN Bits in TCP Header Source port address 16 bits Destination port address 16 bits Sequence Number 32 bits Acknowledgement Number 32 bits HLEN 4 bits Reserved 6 bits URGURG ACKACK PSHPSH RSTRST SYNSYN FINFIN Window size 16 bits Checksum 16 bits Urgent pointer 16 bits Options (if any) Reserved (6 bits)URGACKPSHRSTSYNFIN Reserved (4 bits)CWRECEURGACKPSHRSTSYNFIN CWR: Congestion Window Reduced  Set after TCP sender has reacted to ECN ECE: ECN-Echo  Set when TCP receiver wants to inform the sender of congestion

10. ECN Negotiation in TCP TCP Sender TCP Receiver ECN-Setup SYN CWR = 1 ECE = 1 SYN = 1 ECN-Setup SYN-ACK CWR = 0 ECE = 1 ACK = 1 SYN = 1 ACK ACK = 1

11. Incipient congestion detected. If ECT, then set CE. Example ECN-Capable sender ECN-Capable router ECN-Capable receiver ECN capability negotiated during TCP connection establishment ECT IPTCP Congestion!! TCP sender halves cwnd and sets CWR in TCP-PCI. Congestion!! IP receiver tells TCP receiver. TCP receiver informs TCP sender. Sender has reduced cwnd, TCP receiver stops ECE flag CWR IPTCP CWR IPTCP CE IPTCP ECE IPTCP IPTCP ECE IPTCP IPTCP ECTECN Capable Transport CECongestion Experienced CWRCongestion Window Reduced ECEECN-Echo

12. Rules No ECT set on SYN, SYN-ACK or “pure” ACK PDUs No ECT set unless one ECN-setup SNY or ECN-setup SYN-ACK PDUs has been sent and received No ECT set for retransmitted PDUs Keep the same ECN flags in reassembly when all fragments carry the same ECN set In TIME-WAIT or CLOSE sate, hosts ignore any ECN signaling for that connection …

13. Advantages of ECN Feedback to the TCP sender before the router drops packets Less retransmission Improve throughput and goodput of the whole internetwork

14. Implementations ECN support in TCP by hosts Windows 7, Mac OS X 10.5, Linux 2.6, others? Many modern implementations of the TCP/IP protocol suite have support for ECN; however, ECN is disabled by default ECN support in IP by routers Cisco IOS routers with IOS Release 12.2(8)T, others?

15. Summary Incipient congestion is detected Routers are no longer limited to packet drops as an indication of congestion ECT and CE flags are used in IP-PCI for signaling between routers and hosts ECN-Echo and CWR flags are used in TCP-PCI for host-to-host signaling

16. Reference RFC 3168 – The Addition of Explicit Congestion Notification (ECN) to IP RFC 2309 – Active Queue Management http://en.wikipedia.org/wiki/Explicit_Congestion_Notification#Impleme ntations http://en.wikipedia.org/wiki/Explicit_Congestion_Notification#Impleme ntations http://en.wikipedia.org/wiki/Network_congestion#Active_Queue_Mana gement_.28AQM.29 http://en.wikipedia.org/wiki/Network_congestion#Active_Queue_Mana gement_.28AQM.29 Slides from Guna Ranjan, Justin Yackoski, Namratha Hundigopal and Preethi Natarajan R. Kinicki and Z. Zheng, “A Performance Study of Explicit Congestion Notification (ECN) with Heterogeneous TCP Flows,” http://web.cs.wpi.edu/~rek/ICN01talk.ppt http://web.cs.wpi.edu/~rek/ICN01talk.ppt

17. Q & A

18. AQM Router Mechanism 1 0 Average Queue Length Min-threshold Max-threshold Dropping/Marking Probability Queue Size max_p

19. ECN Goodput Improvement RED: an AQM technique indicating congestion by packet drops ECN performs better